Laminar polymeric sheet

ABSTRACT

A sheet laminate of polyimide and amorphous polyamide especially suitable for making anisotropically electrically conductive articles by laser abiation drilling followed by metal plating. Preferred polyimides include those derived from polymerization of 4,4&#39;biphenyldianhydride and 4,4&#39;-diaminobiphenylether or p-phenylenediamine. Preferred amorphous polyamides include those derived from condensation of terephthalic acid with isomers of trimethylhexamethylenediamine.

This invention relates to a laminar polymer sheet and to products madefrom such a sheet.

Published EP-A-0213774 describes methods of making uniaxiallyelectrically conductive articles by making through-holes in a laminarpolymer sheet, plating metal in the holes, and removing surface laminaefrom the sheet to leave the plated metal in each through-hole projectingbeyond the main surfaces of the sheet. These conductive articles arecommerically valuable as compliant electrical connection interfaces, forexample for connecting microcircuit chips to other elements in anelectronic device. The through-holes can be made conveniently small(preferably not more than 200 micrometers diameter) and may beclose-packed so that random positioning of the article between opposedconnection sites produces electrical connection when the sites arebrought into contact with the opposite main surfaces of the article.Other forms of such articles are described in copending British PatentApplications 8802567, 8802565, 8802568, 8815447.1, 8819895.7, and8823053.7, the disclosures of all of which are incorporated herein byreference.

In view of the wide use of polyimide materials in microelectronicdevices, it is desirable to use polyimides for the uniaxially conductivearticles, but this leads to problems in selecting suitable materials forthe removable surface laminae.

The present invention relates to a novel laminar structure which isuseful for various purposes, and is especially advantageous for makingthe aforementioned uniaxially conductive articles.

The invention accordingly provides a laminar sheet comprising a layer ofpolyimide material laminated in direct contact with a layer ofamorphous, preferably amorphous aromatic, polyamide material.

It will be understood that references to sheets include elongatetape-like sheets and other forms of substantially continuous laminarstructures, in which individual laminae are preferably homogeneous. Forthe purpose of making the aforesaid uniaxially electrically conductivesheets, it is preferable to use a sheet with each of the two mainsurfaces of the layer of polyimide material laminated in direct contactwith a layer of aromatic polyamide material. It may be convenient inmany cases that the polyimide layer is substantially co-extensive withthe polyamide layer(s).

References to the respective layers being "laminated" do not imply anyparticular method of making the laminar structure. Solvent casting ormelt coating of one material onto the other may be used, for example, asan alternative to lamination of two pre-existing solid films, providedthat the desired laminar structure is achieved. For solvent casting,many solvents and solvent blends can be used including, for example:

n-methyl pyrrolidone/10% methanol blend

methanol/chloroform blend

dimethyl formamide (DMF)

dimethyl acetamide (DMAC).

If desired, small quantities of additives (e.g. UV absorbingchromophores such as aromatic carbonyl compounds, e.g. benzophenone) canbe included.

Preferred amorphous polyamides include aliphatic/aromatic polyamides,

(A) polyamides based on the condensation of terephthalic acid withtrimethylhexamethylene diamine (preferably containing a mixture of2,2,4- and 2,4,4-trimethylhexamethylene diamine isomers),

(B) polyamides formed from the condensation of one or morebisaminomethylnorbornane isomers with one or more aliphatic,cycloaliphatic or aromatic dicarboxylic acids e.g. terephthalic acid andoptionally including one or more amino acid or lactam e.g.epsilon-caprolactam comonomers,

(C) polyamides based on units derived from laurinlactam, isophthalicacid and bis-(4-amino-3-methylcyclohexyl) methane,

(D) polyamides based on the condensation of 2,2-bis-(p-aminocyclohexyl)propane with adipic and azeleic acids, and polyamides based on thecondensation of trans cyclohexane-1,4-dicarboxylic acid with thetrimethylhexamethylene diamine isomers mentioned above.

(E) polyamides based on units derived from m-xylylenediamine and adipicacid.

Other preferred polyamides include those based on polyether andpolyamide blocks, especially the so called "polyether-ester amide blockcopolymers" of repeating unit: ##STR1## wherein A represents a polyamidesequence of average molecular weight in the range of from 300 to 15,000,preferably from 800 to 5000; and B represents a linear or branchedpolyoxyalkylene sequence of average molecular weight in the range offrom 200 to 6000, preferably from 400 to 3000.

Preferably the polyamide sequence is formed from alpha,omega-aminocarboxylic acids, lactams or diamine/dicarboxylic acidcombinations having C₄ to C₁₄ carbon chains, and the polyoxyalkylenesequence is based on ethylene glycol, propylene glycol and/ortetramethylene glycol, and the polyoxyalkylene sequence constitutes from5 to 85%, especially from 10 to 50% of the total block copolymer byweight. These polymers and their preparation are described in UK PatentSpecifications Nos. 1,473,972, 1,532,930, 1,555,644, 2,005,283A and2,011,450A, the disclosures of which are incorporated herein byreference.

The polymer preferably has a C:H ratio of not more than 0.9, morepreferably not more than 0.75, most preferably not more than 0.65 andespecially not more than 0.55.

It is notoriously difficult to cross-link polyamides, but it has beenfound unexpectedly that these amorphous polyamides can be cross-linkedby exposure to laser light at power levels which are insufficient toperforate or ablate the polymer. At higher powers, exposure to laserlight can be used to perforate or ablate the polymer to producethrough-holes for making the uniaxially conductive article as aforesaid.Laser light at a wavelength greater than 248 nanometers is preferred,preferably from an excimer laser, especially a KrF excimer laser (249nm) for perforation, or an XeCl excimer laser (308 nm) for crosslinking.

The methods, material and dimensions described in the aforementionedEP-A-0213774 can be used to form through-holes and to put electricallyconductive material in the holes, to produce sheets having through-holescontaining electrically conductive material which provides anelectrically conductive path between the main surfaces of the sheet.Preferably, the electrically conductive material providing theconductive path comprises metal plated on the interior surface of thethrough-holes, and preferably each such conductive path is electricallyseparate from substantially all the other such paths. The inventionincludes the preferred uniaxially electrically conductive articleproduced by removal of at least part of the polyamide layer(s) from thesurface(s) of such a sheet so as to leave the electrically conductivematerial in the through-holes projecting beyond the main surface(s) ofthe sheet. Substantially complete removal of the polyamide layer(s) ispreferred. The disclosures of EP-A-0213774 and of the aforesaidcopending application are accordingly incorporated herein by reference.

It is an advantage of this invention that the polyamides adheretenaciously to the polyimide, are readily and cleanly laser-drillable atthe preferred wavelengths, and are readily removable by suitablesolvents when desired.

The unexpectedly good adhesion of the polyamides to the notoriouslydifficult-to-adhere polyimides enables the laminar sheet to be used asan electrically insulating covering of a wire, with advantages overknown polyimide-insulated wires.

Preferred polyimides include, for example, those commonly used inprinted circuit boards or other components of electronic devices; thosedescribed in EP-A-0178185, the disclosure of which is incorporatedherein by reference; and especially those resulting from polymerisationof 4,4'-biphenyldianhydride and (4,4'-diaminobiphenyl,4,4'-diaminobiphenylether or phenylene diamine), preferablyp-phenylenediamine.

The laminate sheet may carry at least one further layer of polymericmaterial overlying at least one layer of polyamide material, and/or maycarry at least one metallic layer overlying at least one polyamidelayer. For example, acrylates, methacrylates, cellulose esters or otherlacquers, mould release agents, and any other polymer coatings may beused as the further layer provided the material used has adequateadhesion to the polyamide.

A sheet comprising the layer of polyimide material having a layer of thepolyamide material on one or both of its main surfaces, and a metalliclayer on the, or one, or both, polyamide layer(s) may be especiallyuseful in making flexible circuitry.

Specific examples of the invention will now be described by way offurther illustration.

EXAMPLE 1 Production of a Laminate

Three laminates, respectively consisting of a sheet of the polyimidematerials mentioned above derived from 4,4'-biphenyldianydride and (1a)4,4'-diaminodiphenyl (1b) 4,4'-diaminobiphenylether, (c)p-phenylenediamine coated on both sides with amorphous aromaticpolyamide (A) were prepared by solution coating. A solution,approximately 20% by weight, was obtained by dissolving the polyamide inDMF.

A layer of the polyamide (A) hereinbefore mentioned in DMF was coatedonto one side of the polyimide by doctor blading (other methods can alsobe used, e.g. spiral bar coating, spin coating). It was noted that therewas no tendency of the single-sided laminate to curl on drying. Once thefilm was dry to the touch, the other side was coated in a similarmanner. The completed three-layer laminate was subsequently dried in anoven at 150° C. for one hour to remove all traces of residual solvent.The final polyamide thickness can be controlled by the polymer solutionconcentration and the height of the doctor blade.

The finished laminate was then assessed for adhesion by peel testing.The adhesive bond was found to be so good that it was impossible toseparate the polyamide layers from the polyimide without tearing thepolyamide film. Similar results were achieved using the polyamide (C)hereinbefore mentioned instead of polyamide (A).

In contrast, a film of polyetherimide (Ultem, a General Electricpolymer) cast from dichloromethane onto the polyimide layer showedsurprisingly poor adhesion, despite the apparent chemical similarity ofthe two polymers. The whole coated film peeled and completelydelaminated a short time after drying, even without heating.

EXAMPLE 2 Wire Coating

Primary wires insulated with polyamide/polyimide composite can beproduced by a variety of means:

(a) A three-layer polyamide/polyimide/polyamide laminate, produced asdescribed in Example 1, can be cut into strips about 1 cm wide, andthese strips spirally wrapped round conventional conductors (with someoverlap) using well-known means. The insulated wire is then heatedbeyond the softening temperature of the polyamide to consolidate theinsulation, and bind the wraps together.

(b) A copper conductor may be spirally wrapped with polyimide tape, dipcoated with a solution of amorphous aromatic polyamide, and passedthrough a sizing die. The wire is then passed through a heated dryingzone to remove excess solvent, producing a finished insulated primarywire.

(c) A copper conductor may be spirally wrapped with polyimide tape, andthen passed through the head of a conventional extruder to extrude alayer of aromatic polyamide onto the wrapped conductor, and cooled in awater bath to finish the wire.

EXAMPLE 3 Adhesive Layer

A solution of amorphous polyamide as described in Example 1 was used toform an adhesive layer between two pieces of polyimide film. Thesolution was applied to the film using a brush, though the coatingtechniques described in Example 1 could also be used. The second layerof polyimide was placed in contact with the wet adhesive layer and the`sandwich` pressed together and heated to dry the solution. The sandwichwas baked to remove traces of residual solvent and a bonded laminate ofpolyimide to polyimide was formed via the amorphous aromatic polyamide.This process can be repeated to form a multi-layer structure, whichcould subsequently be machined to shape. As an example of machining, amulti-layer sandwich was drilled with an ultraviolet excimer laser toproduce 100 micron diameter through-holes.

EXAMPLE 4 UV Excimer Laser Crosslinking

A wire insulated with polyamide/polyimide composite was prepared asdescribed in Example 2(a), and subsequently crosslinked by UV excimerlaser light. This was achieved by irradiating the wire with laser lightfrom a Lambda Physik XeCl excimer laser (wavelength=308 nm). This laserproduces pulses of UV light of about 30 ns duration, each pulsecontaining about 300 mJ. The laser beam was shaped using lenses andmirrors of types well known in the art so that lengths of wire wereirradiated on all sides at an incident fluence of about 10 mJ/cm² /pulsefor about 20 pulses. Because the laser can be pulsed repetitively at 100Hz or more, it is possible to build a wire handling system that passeswire continuously through the crosslinking zone.

The polyamide/polyimide-insulated wire so produced was compared withsimilar wire that had not been laser treated. Lengths of the insulatedwires were placed in warm dimethylformamide (DMF). It was found that thearomatic polyamide part of the un-treated wire rapidly dissolved,leading to delamination of the insulation. In contrast, thelaser-treated sample demonstrated superior solvent resistance.Microscopic examination showed that the aromatic polyamide had not beendissolved, indicating that crosslinking had indeed occurred.

EXAMPLE 5 Drilling, Plating & Surface Removal

(a) A laminate produced according to Example 1c, with benzophenone addedas a chromophore at a level of 0.1% based on the weight of polyamide,was subjected to laser ablation using an XeCl excimer laser at awavelength of 308 nm to produce a multitude of through-holes of about 50micrometers diameter, which were electrolessly plated with copper, thennickel, then gold using known electroless plating baths, followed byremoval of the polyamide surface layers from the sheet, all using themethods described in the aforementioned EP-A-0213774.

(b) The laminate used for (a) above was subjected to laser ablationusing an KrF excimer laser at a wavelength of 249 nm to produce a "1:1"pattern of through-holes (as described in the aformentioned copendingBritish application) of about 50 micrometers diameter. The holes wereelectrolessly plated with copper, then nickel, then gold using knownplating baths, followed by removal of the polyamide surface layers fromthe sheet to produce a "1:1" uniaxially electrically conductive articleof the kind described in the aforementioned copending Britishapplication.

(c) The procedures (a) and (b) were repeated to produce respectively a"multi-tube" article as in (a) and a "1:1" article as in (b) but withthrough-holes of only 10 micrometers diameter containing the projectingplated metal. A closely spaced regular "1:1" array of this kind can beespecially useful for making connections to high-density regular arraysof electronic devices.

Preferred polyimide materials for the present invention are those whichare capable of retaining at least 50%, preferably at least 75%, morepreferably at least 85%, of its original elongation after immersion inwater of pH10 at 100° C. for 4 days according to ASTM D882. It will bereadily understood that a sufficiently fully cyclised polyimide havingless than 15%, preferably less than 10%, more preferably less than 5%,or if possible substantially no open imide rings or uncyclised amic acidgroupings may be better able to survive hot alkaline metal platingbaths, which attack known polyimides such as Kapton (TM). The use of theaforesaid preferred polyimides derived from 4,4'-biphenyldianhydride hasbeen found advantageous both in general and for the anisotropicallyelectrically conductive articles described and claimed in theaforementioned EP-A-0213774, which accordingly represents anotherinventive aspect, various features of which will be understood from thefollowing numbered paragraphs.

P1. An anisotropically electrically conductive article comprising porouselectrically insulating polyimide sheet material at least a selectedportion of which has at least 25 substantially non-interconnected poresper square millimeter of its surface, at least a significant proportionof which pores individually contain electrically conductive materialwhich provides an electrically conductive path between, and projectsbeyond at least one of, the main surfaces of the sheet material, eachsuch conductive path being electrically separate from substantially allthe other such conductive paths, and the polyimide being derived frompolymerisation of 4,4'-biphenyl dianhydride and (4,4'-diaminobiphenyl or4,4'-diaminobiphenyl ether, or phenylene diamine) and/or being capableof retaining at least 50%, preferably at least 75%, more preferably atleast 85% of its original elongation after immersion in water of pH10 at100° C. for 4 days according to ASTM D882.

P2. An anisotropically electrically conductive article comprising porouselectrically insulating polyimide sheet material at least a selectedportion of which has at least 25 substantially non-interconnected poresper square millimeter of its surface, at least a significant proportionof which pores are internally plated with electrically conductivematerial, and the electrically conductive material provides anelectrically conductive path between the main surfaces of the sheetmaterial and each such path is electrically separate from substantiallyall the other such paths, and the polyimide being derived frompolymerisation of 4,4'-biphenyl dianhydride and (4,4'-diaminobiphenyl or4,4'-diaminobiphenylether, or phenylene diamine) and/or being capable ofretaining at least 50%, preferably at least 75%, more preferably atleast 85% of its original elongation after immersion in water of pH10 at100° C. for 4 days according to ASTM D882.

P3. An anisotropically electrically conductive article comprising porouselectrically insulating polyimide sheet material at least a selectedportion of which has a plurality of substantially non-interconnectedpores at least a significant proportion of which pores individuallycontain a tubular first portion of electrically conductive material,which tubular material projects beyond at least one of the main surfacesof the sheet material, and the polyimide being derived frompolymerisation of 4,4'-biphenyl dianhydride and (4,4'-diaminobiphenyl or4,4'-diaminobiphenylether, or phenylenediamine) and/or being capable ofretaining at least 50%, preferably at least 75%, more preferably atleast 85% of its original elongation after immersion in water of pH10 at100° C. for 4 days according to ASTM D882.

P4. An article according to paragraph 3, wherein the electricallyconductive material provides an electrically conductive path between themain surfaces of the sheet material and each such path is electricallyseparate from substantially all the other such paths.

P5. An article according to paragraph 2, wherein the conductive materialprojects beyond at least one of the main surfaces of the sheet material.

P6. An article according to any preceding paragraph, wherein the sheetmaterial has 25 to 2000 pores per square millimetre in the selectedportion of its surface.

P7. An article according to any preceding paragraph, wherein theconductive material in at least some of the said pores comprises atubular first portion of electrically conductive material in contactwith the pore interior surface, and a second portion of material incontact with the interior surface of the tube provided by the firstportion.

P8. An article according to paragraph 7, wherein the second portion ofmaterial is also tubular.

P9. An article according to paragraph 7 or 8, wherein the material ofthe said second portion is different from the electrically conductivematerial of the said first portion.

P10. An article according to paragraph 7, 8, or 9, wherein electricallyconductive metal, fusible alloy or solder is within the tube provided bythe tubular electrically conductive material.

P11. An article according to any preceding paragraph, wherein theconductive material in at least some of the said pores comprises a firstportion of electrically conductive material in contact with the interiorpore surface, and a second portion of electrically conductive materialon at least one of the end surfaces of the first portion, the secondportion projecting beyond at least one of the sheet surfaces.

P12. An article according to any preceding paragraph, wherein the saidpores have a toruosity less than 3, preferably less than 1.2.

P13. An article according to any preceding paragraph, whereinelectrically insulating material has been removed from one or both mainsurfaces of the sheet material to expose portions of the electricallyconductive material originally within the pores.

P14. An article according to any preceding paragraph, wherein theelectrically insulating sheet material includes a closed cell porousstructure in addition to the said pores containing the electricallyconductive material.

P15. A method of making an article according to any paragraph 7 to 11,comprising:

(a) applying the said first portion of electrically conductive materialto the interior surface of the pores in at least a selected portion ofan appropriate porous electrically insulating sheet material,

(b) removing any electrically conductive material from at least selectedareas of the opposed main surfaces of the sheet material, and

(c) applying the said second portion of material to at least part of thesurface of the first portion.

P16. A method of making an article according to paragraph 13,comprising:

(a) applying the electrically conductive material to the interiorsurface of the pores in at least a selected portion of an appropriateporous electrically insulating sheet material, and

(b) removing electrically insulating material from one of both mainsurfaces of the sheet material so as to expose portions of theelectrically conductive material originally within the pores.

P17. An electrical device or assembly process in which one or moretemporary or permanent electrical connections are made by contact withthe opposite ends of the electrically conductive material located in thepores of an article according to any of paragraph 1 to 14.

P18. A process for testing semiconductive integrated circuit chips,wherein testing circuitry is temporarily electrically connected to anunbonded chip by contact of connection sites of the testing circuitryand the chip respectively with opposite ends of the electricallyconductive material located in the pores of an article according to anyof paragraph 1 to 14.

P19. An article, method, device, or process according to any ofparagraphs 1 to 18, wherein the polyimide is derived from4,4'-biphenyldianhydride and p-phenylenediamine.

The currently more preferred commercially available polyimides are thoseavailable under the Trade Mark "UPILEX" from Ube/ICI. One of these,"UPILEX R", is believed to be a relatively completely cyclised polymerhaving a repeat unit derived from biphenyl dianhydride anddiaminodiphenylether, viz. ##STR2##

Most preferred, however, is "UPILEX S", which is believed to have arepeat unit derived from the same anhydride and phenylene diamine, viz.##STR3##

The polyimide derived from the biphenyldianhydride and4,4'-diaminobiphenyl may have thermal expansion characteristics whichare particularly well suited to microcircuit requirements. Thecorresponding polymers derived from isomers of the diamines mentionedabove, e.g. the 3,4'- or 3,3'-diamino isomers may also be useful, as maythe corresponding polymers derived from pyromellitic dianhydride insteadof the biphenyldianhydride (but excluding Kapton, of course).

For the through-holes previously mentioned, laser drilling, preferablyby ablative photodecomposition using a U.V. excimer laser, has theadvantage of producing through-holes with less pronounced taper thanalternative chemical etching methods, the lower degree of taperpermitting closer pitch (hole-to-hole spacing). This is clearlyadvantageous, given that microcircuits, with which the resultinganisotropically conductive (after metal plating of the through-holes)sheets may be used, are becoming progressively smaller and more denslypatterned. Through-holes with taper (measured on the substantiallystraight inner portions of the holes) less than 10°, preferably lessthan 8°, more preferably less than 6°, and especially less than 4°(relative to the axis of the through-hole) can advantageously beachieved by laser drilling of the laminates according to the presentinvention. This is especially useful for the generally preferred holesof less than 200 micrometers diameter, e.g. 5 to 150 micrometers or 10to 100 micrometers, and especially less than 50 micrometers diameter.

We claim:
 1. A laminar sheet comprising a layer of polyimide materiallaminated in direct contact with a layer of amorphous polyamidematerial.
 2. A sheet according to claim 1 wherein the polyamide materialcomprises amorphous aromatic polyamide material.
 3. A sheet according toclaim 1, wherein each of the two main surfaces of the layer of polyimidematerial is laminated in direct contact with a layer of the polyamidematerial.
 4. A sheet according to claim 1, wherein the polyimide layeris substantially co-extensive with the polyamide layer(s).
 5. A sheetaccording to claim 1, which has been cross-linked by exposure to UVlight.
 6. A sheet according to claim 5 which has been selectivelycross-linked to form a resist pattern.
 7. A sheet according to claim 1,in which through-holes have been produced by exposure of selected areasof the sheet to U.V. laser light.
 8. A sheet according to claim 1,having through-holes containing electrically conductive material whichprovides an electrically conductive path between the main surfaces ofthe sheet.
 9. A sheet according to claim 8, wherein the electricallyconductive material providing each conductive path comprises metalplated on the interior surface of the through-holes.
 10. A sheetaccording to claim 8, wherein each such conductive path is electricallyseparate from substantially all the other such paths.
 11. A sheetproduced by removal of at least part of the polyamide layer(s) from thesurface(s) of the polyimide layer of a sheet according to claim 8, so asto leave the electrically conductive material in the through-holesprojecting beyond the resulting main surface(s) of the sheet.
 12. Asheet according to claim 11 following substantially complete removal ofthe polyamide layer(s).
 13. A sheet according to any of claim 1, in theform of an electrically insulating covering of an electrical wire.
 14. Asheet according to claim 1, wherein the polyamide is derived from(A) thecondensation of terephthalic acid with at least one isomer oftrimethylhexamethylene diamine (preferably containing a mixture of2,2,4-and 2,4,4-trimethylhexamethylene diamine isomers), or (B) thecondensation of one or more bisaminomethylnorbornane isomers with one ormore aliphatic, cycloaliphatic or aromatic dicarboxylic acids andoptionally includes one or more amino acid or lactam e.g.epsilon-caprolactam comonomers, or (C) laurinlactam, isophthalic acidand bis-(4-amino-3-methylcyclohexyl) methane, (D) the condensation of2,2-bis-(p-aminocyclohexyl) propane with adipic and azeleic acids, orthe condensation of trans cyclohexane-1,4-dicarboxylic acid with thetrimethylhexamethylene diamine isomers mentioned in (A) above, or (E)polyamides based on units derived from m-xylylenediamine and adipicacid.
 15. A sheet according to claim 1, wherein the polyimide is derivedfrom 4,4'biphenyldianhydride and (4,4'-diaminobiphenyl, or4,4'-diaminobiphenylether, or phenylenediamine).
 16. A sheet accordingto claim 1, comprising at least one further layer of polymeric materialoverlying at least one layer of the polyaminde material.
 17. A sheetaccording to claim 1 comprising at least one metallic layer overlying atleast one layer of polyamide material.
 18. A sheet according to claim 17comprising the layer of polyimide material having a layer of thepolyamide material on one or both of its main surfaces, and a metalliclayer on the, or one, or both of the said further layers.
 19. A sheetaccording to claim 1, wherein the polyimide material is capable ofretaining at least 50%, preferably at least 75%, more preferably atleast 85% of its original elongation after immersion in water of pH10 at100° C. for 4 days according to ASTM D882.
 20. A sheet according toclaim 1, wherein the polyimide material is derived from polymerisationof 4,4'-biphenyl dianhydride and p-phenylenediamine.
 21. Ananisotropically electrically conductive article comprising porouselectrically insulating polyimide sheet material at least a selectedportion of which has at least 25 substantially non-interconnected poresper square millimeter of its surface, at least a significant proportionof which pores individually contain electrically conductive materialwhich provides an electrically conductive path between, and projectsbeyond at least one of, the main surfaces of the sheet material, eachsuch conductive path being electrically separate from substantially allthe other such conductive paths, and the polyimide being derived frompolymerisation of 4,4'-biphenyl dianhydride and (4,4'-diaminobiphenyl,or 4,4'-diaminobiphenylether, or phenylenediamine) and/or being capableof retaining at least 50%, preferably at least 75%, more preferably atleast 85% of its original elongation after immersion in water of pH10 at100° C. for 4 days according to ASTM D882.
 22. An anisotropicallyelectrically conductive article comprising porous electricallyinsulating polyimide sheet material at least a selected portion of whichhas at least 25 substantially non-interconnected pores per squaremillimeter of its surface, at least a significant proportion of whichpores are internally plated with electrically conductive material, andthe electrically conductive material provides an electrically conductivepath between the main surfaces of the sheet material and each such pathis electrically separate from substantially all the other such paths,and the polyimide being derived from polymerisation of 4,4'-biphenyldianhydride and (4,4'-diaminobiphenyl, or 4,4'-diaminobiphenylether, orphenylenediamine) and/or being capable of retaining at least 50%,preferably at least 75%, more preferably at least 85% of its originalelongation after immersion in water of pH10 to 100° C. for 4 daysaccording to ASTM D882.
 23. An anisotropically electrically conductivearticle comprising porous electrically insulating polyimide sheetmaterial at least a selected portion of which has a plurality ofsubstantially non-interconnected pores at least a significant proportionof which pores individually contain a tubular first portion ofelectrically conductive material, which tubular material projects beyondat least one of the main surfaces of the sheet material, and thepolyimide being derived from polymerisation of 4,4'-biphenyl dianhydrideand (4,4'-diaminobiphenyl, or 4,4'-diaminobiphenylether, orphenylenediamine) and/or being capable of retaining at least 50%,preferably at least 75%, more preferably at least 85% of its originalelongation after immersion in water of pH10 at 100° C. for 4 daysaccording to ASTM D882.
 24. A sheet or article according to claim 1having through-holes laser-drilled therein with a hole taper (relativeto the hole axis) less than 10°, preferably less than 8°, morepreferably less than 6°, and especially less than 4°.